A method of exposing photosensitive glass to light and thermally developing it has been heretofore known. The developed glass may be etched. For example, this method is described by Takashi Matsuura in his paper entitled, "Photo Chemically Machinable Glass Ceramics", Practical Surface Technologies, No. 11, pp. 1-7, 1988. This is hereinafter referred to as Reference 1. This method comprises an exposure step, a thermal development step, and an etching step. In the exposure step, desired portions of photosensitive glass are exposed to light from an ultraviolet lamp such as an extra-high pressure mercury lamp. In the thermal development step, the glass is heated to 500.degree. to 700.degree. C. to crystallize the exposed portions. In the etching step, the crystallized, exposed portions are dissolved away by an etchant (solution of hydrofluoric acid).
Even if the method ends with the thermal development step without carrying out the etching step, a crystalline portion formed in the glass appears to be colored yellow or brown because of the light-transmitting characteristics of the crystalline portion. Therefore, this glass can be used for displaying or recording purposes.